Explain how a skeletal muscle fibre relaxes after stimulation ceases.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
1. Action potentials at the neuromuscular junction stop, so no further acetylcholine is released (or it is hydrolysed by acetylcholinesterase), preventing depolarisation of the sarcolemma and T‑tubules.
2. The sarcoplasmic reticulum actively pumps Ca²⁺ back into its lumen via Ca²⁺‑ATPase pumps, using ATP, which lowers the cytoplasmic Ca²⁺ concentration.
3. With low Ca²⁺, troponin releases the bound calcium and reverts to its original conformation, allowing tropomyosin to cover the myosin‑binding sites on actin.
4. The blocked binding sites mean myosin heads cannot attach to actin, cross‑bridge cycling stops and the thin filaments (titin and connective tissue) return the sarcomere to its resting length.
2. The sarcoplasmic reticulum actively pumps Ca²⁺ back into its lumen via Ca²⁺‑ATPase pumps, using ATP, which lowers the cytoplasmic Ca²⁺ concentration.
3. With low Ca²⁺, troponin releases the bound calcium and reverts to its original conformation, allowing tropomyosin to cover the myosin‑binding sites on actin.
4. The blocked binding sites mean myosin heads cannot attach to actin, cross‑bridge cycling stops and the thin filaments (titin and connective tissue) return the sarcomere to its resting length.
Examiner tips
- Use the word ‘stop’ or ‘cease’ to show the action potential ends. Mention acetylcholinesterase to show the mechanism of ACh removal. Show the ATP‑dependent SR Ca²⁺‑ATPase pump. Explain the troponin‑tropomyosin block and the resulting cessation of cross‑bridge cycling.
Common mistakes
- Forgetting that acetylcholinesterase breaks down ACh. Saying the SR simply ‘takes up’ Ca²⁺ without noting ATP‑dependent pumps. Omitting the role of tropomyosin blocking the myosin‑binding sites.
Mark scheme (4 marks)
- Action potentials at the neuromuscular junction cease, so no further acetylcholine is released (or acetylcholine is broken down by acetylcholinesterase), stopping depolarisation of the sarcolemma / T-tubules.
- The sarcoplasmic reticulum actively pumps calcium ions (Ca²⁺) back into its lumen using ATP / Ca²⁺-ATPase pumps, lowering cytoplasmic Ca²⁺ concentration.
- With low cytoplasmic Ca²⁺, troponin releases calcium ions and reverts to its original conformation, causing tropomyosin to move back and block the myosin-binding sites on actin.
- With binding sites blocked, myosin heads cannot attach to actin and cross-bridge cycling stops; the thin filaments slide back (or elastic recoil of titin / connective tissue) returns the sarcomere to its resting length.
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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